The Invention concerns a rear projection screen with a translucent image-forming surface arranged between a projector and the viewer.
The image-forming surface serves to render visible a virtual image produced by still or motion picture projectors. In order to permit observation of the image from the side facing away from the projector, the rear projection screen must be light-transmitting and produce light diffusion.
Rear projection screens are known in the field of advertising and information in numerous ways. They can consist according to DE 80 26 773 U1 in their simplest form of a sand-blasted ground-glass screen. Their maximum possible dimensions are restricted by structural considerations. That is to say, large-area rear projection screens can in principle be set up where the image-forming surface consists of several ground-glass screens arranged above another and/or side-by-side. Such rear projection screens have not however become known hitherto. They could only be produced at considerable structural expense. The wind load acting on such a structure would necessitate such a robust supporting structure that the quality of the image projection would be significantly impaired. In addition, imparting a mat finish by sand-blasting reduces the strength of glass, which, in the case of large-area rear projection screens consisting of numerous sand-blasted ground-glass screens, would additionally lead to stability problems.
The purpose of the Invention is to provide a large-area rear projection screen which can be used outdoors, permits image projection of sufficiently high quality and can be produced with little expense and onto which one can project in front of a building, for example a large movie theater, images which are recognizable from a long distance.
According to the Invention, provision is made for the image-forming surface to be formed by bases possessing light-diffusing properties of a number of U-shaped profile elements arranged side-by-side.
When setting the light-diffusing properties of the bases of the U-shaped profile elements within the scope of the Invention, it is possible to resort to the general knowledge of rear projection screens. Preferably, the light transmission TL of the bases (with vertical light incidence) will be at least approximately 50% with a proportion of diffused light of the light transmitted sufficiently high to permit rear projection. If viewing of the image produced is to be permitted from a wide angle, the light diffusion is to be set as uniformly as possible over the entire range from 0 to 180xc2x0 . If, on the other hand, the image should be especially visible from a certain viewing position, it is to be ensured that the light diffusion effect in the direction concerned is higher than in other directions.
Of course, the bases need not possess the same light-diffusing properties over their entire width and height. On the contrary, is will suffice if the predominant portion of the bases are provided with light-diffusing properties.
Preferably, the U-shaped profile elements will consist of glass. U-shaped profile glass elements usually consist of alkali silicate glass whose U-shaped profile is imparted to it like cast glass in the machine rolling process. It is translucent, with an irregular or ornamental surface produced by the production process and possesses on the whole quality features typical of cast glass. The light transmission for commercial, U-shaped profile glass elements laid in a single layer without light-diffusing properties is up to 89%; for U-shaped profile glass elements laid in two layers, it is up to 81%. Of course, the light diffusion of the bases of the U-shaped profile glass elements according to the Invention must be significantly higher than in the case with commercial U-shaped profile glass elements with or without ornamentation.
By light-diffusing properties within the scope of the Invention is meant that only a small proportion, preferably less than 10%, of the transmitted light experiences no or only slight deflection. The structures suitable for rear projection (surface roughening, frosting, light-diffusing coatings, internal structures or the like) must possess dimensions in the micron range. These are typically in the range from 1 to 100 microns, preferably approximately 50 microns. Such textured surfaces produce a mat effect in incident light and are practically opaque with high intensity of the incident light.
Commercial U-shaped profile glass elements are manufactured in a thickness of 6 or 7 mm, with a length of up to 7000 mm, with a base width of 232 to 498 mm and with a flange height of 41 or 60 mm. U-shaped profile glass elements are especially weather-resistant and insensitive to corrosive air in industrial and coastal regions.
The use of U-shaped profile glass elements with increased light-diffusing action for construction of building walls is already known from DE 17 84 056 A1. There, for the purpose of thermal insulation, the U-shaped profile glass elements are combined with spun glass or capillary sheets or films with high reflectivity and light filtration. These measures are additionally intended to produce sufficient diffusion of the incident light from outside such that uniform interior illumination is achieved with daylight.
Surprisingly, the flanges of the U-shaped profile elements, which are usually aligned perpendicular to the bases and thus essentially parallel to the direction of projection, disturb only insignificantly on implementation of the Invention. Of course, in the case of U-shaped profile elements utilizable within the scope of the Invention, the thickness of the flanges must be significantly less than the width of the bases, as only in this way is sufficiently good image quality achievable. Preferably, U-shaped profile elements will be used where the flange thickness is a maximum of approximately {fraction (1/10)} of the width of the base.
In practical application, the U-shaped profile elements will be normally installed such that their flanges are facing towards the projector. In this way, the surface of the rear projection screen facing the viewer located outside the building will be easy to clean and aesthetically pleasing.
With the Invention, it is possible to construct rear projection screens of practically any surface dimensions. By using a number of U-shaped profile elements which are light-diffusing in the area of their bases, whichxe2x80x94as already describedxe2x80x94consist preferably of glass, it is guaranteed that even large-area projection screens can still possess sufficient stability. According to the Invention, it is even possible, by virtue of the special mechanical properties of U-shaped profile elements, to impart the light-diffusing properties by sand-blasting at least one of the surfaces of the bases.
It lies within the scope of the Invention to use U-shaped profile elements whose bases consist of a light-diffusing material. The light-diffusing effect can be achieved, for example, by embedding very small particles with a refractive index differing from that of the matrix, gas bubbles for example, in the material of the base. Preferably instead however, at least one of the surfaces of the bases will be provided with a coating imparting the visual impression of a sand-blasted surface, or subjected to a frosting treatment. In this way, it is possible, to reduce the cost of manufacture of the rear projection screens, to use commercial U-shaped profile elements which are only modified superficially for the special application. Here, it is of special advantage for the inner surfaces of the bases facing the projector to possess light-diffusing properties. In this way, the rough surface is facing the building interior and thus better protected from contamination than would be the case of arrangement on the exterior.
Although in principle, instead of sand-blast frosting, frosting produced by an etching process would also be possible, there are environmental objections to the latter. Grinding treatment that is also possible in principle would involve high technical expense, as in any case when using glass, it could only be carried out after shaping of the U-shaped profile elements.
Therefore, according to the Invention, another preferred alternative to mechanical treatment of the surface(s) of the bases consists of combining light-diffusion treated substrate materials with the bases. These substrate materials can be films or the like, in which light-diffusing particles are embedded or which are provided with a coating of light-diffusing particles.
Especially preferred of course is to apply light-diffusing varnishing to the base. Varnishing has the advantage that the diffusing property can be regulated in a simple manner by the coating thickness and/or the proportion of the light-diffusing particles in the varnish coating applied. In this way, it is also possible to solve the problem of selective and, where applicable, application-specific control of the light-diffusing effect in a simple manner. Especially suitable are varnishes which incorporate a polyamide-based frosting medium and plastic powder with a particle size of 55-65 microns as light-diffusing particles, such as for example the colorless Pehacryl-2K sand-blasting effect varnishes of Messrs Peter Lacke, Hiddenhausen, Germany.
Above all, from the production aspect, it is advantageous if, in addition, the flanges of the U-shaped profile elements possess light-diffusing properties at least in some areas, and in particular possess the same coating or have been treated in the same way as the bases.